docs: add NG0991 error page and document rxResource's completion contract - #70259

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RESOURCE_COMPLETED_BEFORE_PRODUCING_VALUE had no guide, no JSDoc on
RxResourceOptions.stream, and — since the code was positive rather
than negative — could never get an auto-linked docs page even if one
existed. Flip it to -991, add the NG0991 reference page, and document
the "stream must emit a value or an error before completing"
requirement on stream's JSDoc and in the RxJS interop guide.

Also documents and tests that an unguarded template read of an
errored resource's .value() propagates to the global ErrorHandler,
and recommends guarding with .hasValue() as defense in depth.

httpResource can throw the same error, but for a different reason:
its internal request Observable isn't something app code writes
directly, so an empty completion there is almost always an
HttpInterceptor swallowing the response (catchError(() => EMPTY))
rather than a stream authored in the resource() call itself. The page
covers both APIs with guidance matched to what's actually going on
for each.